1. Field of the Invention
The present invention relates to an energy storage device as a backup power supply for supplying power in its energy storage section to a load when voltage of a primary power supply reduces.
2. Background Art
A primary power supply, which is a DC power supply, is formed of a rectifier circuit for rectifying commercial AC power supply or a car battery. An energy storage device as a backup power supply includes an energy storage element such as a capacitor, and makes up for voltage reduction by supplying power stored in the energy storage element to a load when the voltage of the primary power supply reduces. The load can thus continue its operation. This type of energy storage device is used for covering a momentary voltage drop in the primary power supply or as an emergency power supply. If characteristics of the energy storage element are degraded, the energy storage device may not be able to supply sufficient power to the load at occurrence of voltage drop. Therefore, it is important to determine characteristic degradation in the energy storage element.
In general, if degradation in capacitor characteristics worsens, the storage capacity reduces, and thus capacitance becomes small. Accordingly, if a capacitor is used as the energy storage element, characteristic degradation can be determined by checking the capacitance.
Next, an example of method of measuring capacitance of capacitor is described with reference to FIG. 9. FIG. 9 is a schematic diagram of a capacitor performance (capacitance) measuring instrument. Measuring circuit 100 includes constant current source 110 and oscilloscope 120. Sample 200, which is a capacitor to find capacitance thereof, is connected to constant current source 110. In addition, oscilloscope 120 for monitoring the voltage of sample 200 is connected to both ends of sample 200.
Capacitance C of sample 200 is calculated as follows. If the current of constant current source 110 is I, and voltage of sample 200 that changes by charging during period t is V, the relationship of C·V=I·t is established. Since capacitance C and current I are constant, V=I·t/C is established. Voltage V shows a linear correlation with respect to period t. Accordingly, if sample 200 is charged with constant current by constant current source 110, its voltage linearly changes by time. Using this character, oscilloscope 120 measures a change in voltage V against period t, so as to measure capacitance C based on C=I·t/V. Alternatively, voltage V may be converted and measured in digital data by using an A/D converter, instead of oscilloscope 120.
Capacitance C of capacitor can be measured by using the above measuring instrument. However, it is not realistic to build in the measuring instrument including an oscilloscope to the energy storage device used as a backup power supply. A configuration of collecting digital data of voltage V by an A/D converter can be built in the energy storage device. However, accuracy may be insufficient depending on specifications of energy storage device. Reasons are given below.
In a configuration using an A/D converter instead of oscilloscope 120 in FIG. 9, variation of voltage V with time is measured up to the voltage lower (specifically, 0.3 V) than operating voltage of non-linear element in a circuit including the capacitor. Therefore, the A/D converter that can measure the voltage at least up to 0.3 V is sufficient.
On the other hand, if the A/D converter is a 10-bit A/D converter and a level shifter is also built-in, an assumed configuration is to capture voltage V at 10-bit resolution performance by amplifying input voltage ten times, for example, by the level shifter. In this case, the input voltage up to 5 V can be captured with about 4.9 mV (5V/(210−1)) at full scale. This resolution performance is called 1 LSB.
A general A/D converter has an error of about ±5 LSB, and thus an output error of the above A/D converter is about ±0.49% (±5 LSB×4.9 mV/5000 mV×100). This error achieves a sufficient accuracy as a performance measuring instrument for measuring capacitance C of capacitor.
On the other hand, if a load requiring backup power needs as high voltage as about 50V, for example, and the above general A/D converter is used for detecting high voltage, voltage V that can be captured is still up to 5V. Accordingly, voltage V needs to be input to the A/D converter after reducing one digit from a high-voltage value such as by applying resistive division. A captured voltage accuracy in this case becomes ±5 LSB×4.9 mV=±24.5 mV as described above. Therefore, a capture error is ten times, that is ±245 mV with respect to the high voltage (50V).
If voltage V is calculated under this capture error, an error in voltage V becomes as follows. Voltage V is a difference in absolute voltages at two points before and after period t. For example, if voltage V is 2V, absolute voltage values at two points whose difference is 2V, such as 48V to 50V, must be captured. An error at capturing 48V is, as described above, ±245 mV, and an error at capturing 50V is also ±245 mV. Therefore, an error of voltage V, which is a difference between these values, becomes ±490 mV at the maximum.
In other words, an error may become ±490 mV at the maximum relative to 2V, which is voltage V. In this case, the error reaches ±24.5% (±0.49 V/2V×100). Accordingly, if capacitance C is calculated using this voltage V, the error is large, and thus the determination accuracy of characteristic degradation is inadequate.
If voltage V is increased, the error becomes relatively small. However, period t becomes longer, which means more time is required for determining degradation by calculating capacitance C. Still more, one of absolute voltage values at two points needs to be a small value. If the voltage of primary power supply drops while charging from a low to high absolute voltage values, sufficient backup power many not be supplied.
To increase the measuring accuracy of capacitance C, for example, resolution performance (number of bits) of A/D converter may be increased. However, this makes circuit configuration more complicated.